Transport networks serve for the physical layer transport of high bitrate tributary signals. In particular, the signals transmitted over a transport network are encoded and multiplexed into a continuous bit stream structured into frames of the same length. Within this constant bitrate bit stream, the frames are repeated periodically with a frame repetition rate of typically 8 kHz and are substructured according to a multiplexing hierarchy. An example of such a multiplexing hierarchy is SDH (Synchronous Digital Hierarchy, see ITU-T G.707 October 2000) where the frames are termed synchronous transport modules of size N (STM-N, where N=1, 4, 16, 64, or 256). The frames have a section overhead and contain at least one higher order multiplexing unit called virtual container VC-4, which can either directly carry a tributary signal or a number of lower order multiplexing units like VC-12 or VC-3, which then carry tributary signals.
Virtual containers are transmitted from source to sink through an SDH network and therefore represent a “logical” path through the network. The sequence of identical VCs having the same relative position in subsequent frames forms a traffic stream along that path. Each VC contains a path overhead (POH) and a payload section referred to as a container (C). The US equivalent of SDH is known as SONET (Synchronous Optical Network).
Another well known transport network with similar multiplexing units is the recently defined Optical Transport Network OTN; see ITU-T G.709, February 2001. In the OTN, the transport signals are colored, wavelength multiplexed signals and the multiplexing unit that defines a path is a single wavelength channel thereof. The design of the OTN allows it serve also as a server layer for an SDH transport network.
A transport network itself consists of a number of physically interconnected network elements such as crossconnects, add/drop multiplexers and terminal multiplexers. Traditional transport networks are managed centrally. This means that a central network manager has the overview about the topology and status of the network and if a customer desires a new connection for a tributary signal, the network operator manually establishes via his network management system a corresponding path through the transport network. Thus, paths through a centrally managed network are created under the control of the central network management system, which instructs all affected network elements (potentially using intermediate lower level network management facilities) to setup corresponding crossconnections to establish the new path.
Recent evolution, however, led to the introduction of a distributed control plane and the definition of a related protocol known as GMPLS (Generalized Multi-Protocol Label Switching). The underlying principle is that each network element has its own GMPLS controller. The GMPLS controllers in the network communicate with each other over a dedicated data network, known as the control plane, to find an available route through the network, coordinate path set-up and configure their corresponding network elements accordingly to automatically establish the dynamically agreed path. Each GMPLS controller must therefore have a complete knowledge about the topology and status of its network domain and about gateway nodes to other domains. An OSPF protocol (Open Shortest Path First), slightly extended to the particular needs of a GMPLS-controlled transport network, is used to communicate (or “advertise”) the status of the transport network from one GMPLS controller to the other. Each controller has a database where it stores the topology data of the network according to its latest knowledge.
A very basic aspect in all types of transport networks is availability and reliability of service. In other words, a transport network must be very robust against any kind of failure and the average outage time must be very low. Hence, a transport network needs to provide the means and facilities to ensure sufficient availability. Typically, network mechanisms which ensure this availability are distinguished in protection and restoration. The common principle of both is to redirect traffic of a failed physical link or logical path over a spare resource.
The subtle distinction between restoration and protection is made based on the resource allocation done during the recovery state. Resource allocation means here the active use of a resource, i.e., the resource caries traffic. The recovery state is the status when the traffic is restored over the spare path. For a protection mechanism, the resources are assigned prior to any failure, while for restoration, the resources are assigned only after occurrence of a failure.
Protection is a mechanisms where an already established protection path or link is assigned to one selected high-priority path or link (known as 1+1 or 1:1 protection, depending on whether there is low priority extra traffic on the protection resource or not) or a group of n such selected paths or links (known as 1:n protection). In the case of a failure, traffic can be restored very fast over the previously established protection resource under the sole control of the affected network elements in typically less than 50 ms. However; this requires a protocol between the affected nodes to signal and synchronize switch-over.
Protection is a high-quality service restricted to few selected premium connections, which are typically charged at a higher price. Moreover, protection requires a high amount of spare resources compared with the amount of protected resources, i.e., 100% of spare capacity in the case of 1+1 protection.
Restoration refers to a mechanism, where the network searches for restoration capacity and establishes a restoration path only after a service path failure. Typically, connections are restored upon the occurrence of a failure by setting up a new path and by deleting the failed one. Rather than calculating the restoration path after failure, pre-calculated restoration routes can be used instead but with the actual cross-connection to establish the pre-calculated path performed after failure. Restoration mechanisms are more stringent in the usage of spare capacity and, however, provide a masking of the failure at a lower speed, typically in the range of a few seconds as completely new paths through the network must be established.
In an automatically switched optical transport network, restoration actions are distributed among the entire network. The GMPLS controllers of the affected network elements need to determine from their routing information possible alternate routes and negotiate path set-up with the counterpart controllers along that path.
A problem may arise in GMPLS-controlled transport networks or in multi-layered transport networks, when concurrent restoration actions interfere with each other. In other words, a failure that has happened in a lower network layer may trigger restoration actions in a higher layer but also concurrent restoration actions in the lower layer itself. This may lead to unnecessary and unwanted re-configuration steps and may delay the restoration.